1. School of Engineering and Applied
Sciences, Harvard University, Cambridge,
Massachusetts 02138, United States
2. International Center for Quantum
Materials, Peking University, No. 5 Yiheyuan
Road, Haidian District, Beijing 100871, China
3. Department of Physics, Harvard University, Cambridge, Massachusetts 02138,
United States
4. Institute
of Physics, Chinese Academy of Sciences, Beijing 100190, China